(a) Field of the Invention
The present invention relates to a next generation mobile communications system. More particularly, the present invention relates to a system and method for determining the use of forward channel transmit diversity, that is, STTD (Space-Time block-coding-based Transmit Diversity), encoding in a base station of an asynchronous broadband CDMA (Code Division Multiple Access) system.
(b) Description of the Related Art
Next generation mobile communications systems provide various high speed (2 Mbps) and high quality services such as multimedia capabilities.
With the goal of providing such services, the UTRAN (Universal Terrestrial Radio Access Network) FDD (Frequency Division Duplex) standard was made at the 3GPP (3rd Generation Partnership Project), which was a meeting that took place to create a third generation standard.
In wireless communications systems, there is a reduction in performance caused by a multi-path fading channel. A variety of methods are used to minimize this loss of performance.
Among the various methods, an effective way to prevent this problem is through the use of an antenna diversity technology, in which many antennas are used in a receiver of a base station. The basic reason for such a configuration is economic: a single base station may be used for many mobile stations, while use of a plurality of antennas in mobile stations is costly.
Many methods have been proposed to receive such benefits of diversity. The technology adopted in the 3GPP standard is an open-loop diversity STTD technology. The STTD technology was first proposed in 1998 by S. M. Alamouti (see Standardization Specifications 3GPP 3G TS 25.211).
STTD encoding adopted in the 3GPP standard is optional for a base station. Accordingly, even with an increase in added complexities, mobile stations and terminals must provide the ability to execute STTD demodulation in receivers.
When the power of a mobile station is first turned on, a three-stage cell search unit of a receiving end acquires timing information of a base station cell with the largest signal and a scrambling code number. Next, before system information sent through a primary common control physical channel (PCCPCH), which is a forward channel, is demodulated in the UTRAN, the receiver must compensate frequency offset between the base station and mobile stations. However, it must be known whether STTD encoding of the base station is being used to perform such frequency offset compensation.
It may be determined if STTD encoding of the base station is being used through symbols contained in a synchronization channel (SCH) as shown in FIG. 1.
FIG. 1 is a drawing showing a structure of a PCCPCH according to a 3GPP radio access network (RAN) standard. There is no transmission at an initial point of each slot during a 256-chip interval, and instead a primary SCH and a secondary SCH are transmitted in this interval.
Symbol (a) included in the primary SCH and the secondary SCH indicates whether STTD encoding of the base station is being used. That is, if a value of (a) is +1, STTD encoding is being used, while if the value of (a) is −1, this indicates that STTD encoding is not being used.
However, in this method of determining whether STTD decoding is being used through the symbol included in the SCHs of the PCCPCH, in the case where one base station is using STTD encoding and another is not using STTD encoding, and slot timings of the two base stations overlap, it is not possible to determine whether STTD encoding is being used for the base station cell with the largest reception signal.
Further, since there is no spreading gain if the SCHs do not perform spreading, errors as a result of multi-path fading and frequency offset are significant compared to other channels, thereby increasing the likelihood of making incorrect determinations of whether STTD encoding is being used.